Capacitance is the amount of charge drawn upon a pair of conductors separated by dielectric material for a voltage applied across the pair of conductors. FIG. 1 shows a classic parallel plate capacitor. The classic parallel plate capacitor includes a first conducting plate 101 that is parallel with a second conducting plate 102. The pair of conducting plates 101, 102 are separated by dielectric material (which is not specially depicted for convenience in the drawing of FIG. 1). Each plate 101, 102 may be viewed as a separate node of the capacitor. The capacitance, C, for a classic parallel plate capacitor may be expressed asC=(εA)/d  EQN. 1 where: 1) ε is the permitivity of the dielectric material; 2) A is the cross sectional area of the conducting plates 101, 102 (as measured along the xy plane): and, 3) d is the distance between the conducting plates 101, 102 (as measured along the z axis).
If a voltage is applied across the conducting plates 101, 102 electric flux lines are established between the plates (substantially along the z axis in the depiction of FIG. 1). The density or amount of flux lines is proportional to the charge drawn on the plates. Hence large permitivity ε and large plate area A each correspond to large capacitance. Note also that the closer the parallel plates are spaced apart (i.e., the smaller d becomes), the larger the capacitance. Thus, capacitance is strongly related to the geometry of the capacitor's design as represented by the terms A and d in EQN. 1, above.
Semiconductor manufacturing processes are capable of forming conductors of various shapes and sizes separated by dielectric; and, therefore, are likewise capable of forming a multitude of different types of capacitive structures (e.g., beyond the simple parallel plate capacitor of FIG. 1). However, some types of capacitive structures may be more suitable for certain types of electrical circuits. For example, in the case of certain types of Analog-to-Digital Converter (ADC) circuits, arrays of identically designed capacitors are used. The more that a plurality of capacitors can be designed and manufactured to have the same capacitance (a characteristic referred to as “matching”), the higher the bit resolution that may be targeted for an ADC circuit that uses them (which, in turn corresponds to a more precise analog-to-digital conversion).